The use of radio frequency (RF) signals, such as for providing wireless communication of voice, images, and data, for use in imaging, to provide sensing, etc., is commonplace to the point of nearly becoming ubiquitous. Due to various reasons, such as the availability of relatively unused spectrum, radiation providing penetration of a wide variety of materials, etc., the use of RF signals at higher and higher frequencies has become of interest. For example, the RF frequencies in the millimeter-wave (mmW) band are gaining increasing interest, such as for use with respect to the ever-increasing demands for high-data-rate wireless communication, radar sensing and imaging, etc. Various configurations of mmW transceiver systems useful in one or more of the foregoing applications may, for example, be developed using mainstream complementary metal-oxide-semiconductor (CMOS) technologies, such as due to CMOS devices generally having desirable characteristics including high noise reliability, low static power consumption, and low cost.
A local oscillator (LO) signal is often indispensable in the above transceiver configurations for performing frequency conversion between the RF and the baseband. Generally, the LO frequency is required to be tunable to accommodate various operating channels and the phase noise should be sufficiently low to negligibly deteriorate the signal-to-noise ratio (SNR). Accordingly, the LO signal may be provided by variable frequency oscillator circuit configurations, such as voltage controlled oscillator (VCO) configurations.
To address the performance requirements of mmW VCOs, various topologies and techniques have been developed for mmW LO generation. In particular, directly synthesizing the LO signal based on a VCO oscillating at the desired frequency is currently recognized as one of the optimal choices due to its large output swing, continuous frequency coverage, and the simplicity for system integration. However, circuit design and implementation of a VCO in CMOS suitable for such direct LO signal synthetization remains a major challenge, especially considering that a wide frequency range is typically required to tolerate process, voltage, and temperature (PVT) variations. The quality factor of tuning varactors is predominantly low at mmW frequencies, which directly results in phase noise degradation. Enlarging the VCO transistors and consuming more power could increase the oscillation amplitude and thus improve the phase noise, although the corresponding increase in induced parasitic capacitance would in turn narrow down the tunable frequency range of the VCO. Moreover, if multiple-phase output were to be provided, the situation would worsen due to the additional loading from the coupling devices.
Recently, frequency tuning techniques for mmW VCOs have included techniques based on switched inductors or transformers to vary the effective inductance in the inductance and capacitance (LC) tank and thus to achieve wide frequency tuning range. However, such techniques continue to require low-Q varactors in the VCO to continuously fine-tune the frequency. Moreover, the phase noise at certain frequency ranges is poor, limiting the achievable figure-of-merit (FoM) for VCOs using these frequency tuning techniques. If multi-phase oscillator were provided using these frequency tuning techniques, these techniques would inevitably experience performance degradation due to the more parasitics presented.